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Report on the Progress and Present State of Physical, Optics. 

 By the Rev. Humphrey Lloyd, A.M., M.R.I.A., Fellow 

 of Trinity College, and Professor of JVattcral and Experi- 

 mental Philosojjhy in the University of Dublin. 



In the Report which I have the honour to submit to the Associa- 

 tion, I have attempted to consider in some detail the present 

 state of our knovpledge with regard to the physical theory of 

 light, and the successive advances which have, in late years, 

 been made towards its establishment. The method which I 

 have thought it expedient to adopt in this review has been to 

 take, in the first instance, a rapid survey of the several leading 

 classes of optical phenomena, which the labours of experimental 

 philosophers have wrought out in such rich profusion, and after- 

 wards to examine how far they are reducible to one or other of 

 the two rival theories which have alone advanced any claim to 

 our consideration. This is, in fact, the only way in which the 

 truth of a physical theory can be established ; and the argument 

 in its favour is essentially cumulative. 



But in making this comparison it is not enough to rest in 

 vague explanations which may be moulded to suit any theory. 

 Whatever be the apparent simplicity of an hypothesis, — whatever 

 its analogy to known laws, — it is only when it admits of mathe- 

 matical expression, and when its mathematical consequences 

 can be numerically compared with established facts, that its 

 truth can be fully and finally ascertained *. Considered in this 

 point of view, the wave-theory of light seems now to have 

 reached a point almost, if not entirely, as advanced as that to 

 which the theory of universal gravitation was pushed by the 

 single-handed efforts of Newton. Varied and comprehensive 

 classes of phnomena have been embraced in its deductions; 

 and where its progress has been arrested, it has beeii owing 

 in a great degree to the imperfections of that intricate branch of 

 analysis by which it was to be unfolded. The principles of the 

 theory of emission, on the other hand, have, in comparativelj' 



• C'est en tirant des formules les consequences les plus subtiles et les plus 

 eloign^es des principes, puis allant les verifier par I'experience, que Ton peut 

 reellement s'assurer si une th6orie est vraie ou fausse, et si Ton doit s'y confier 

 conime a uu guide fidele, ou la rejeter comme un syst^me trompeur. — Biol, 

 Traile de Physique, torn. i. p. xiv. 



